Head blight, also known as head scab, pink mold, whiteheads and tombstone scab, is a devastating disease afflicting wheat, barley, and several other cereal crops worldwide, particularly in USA, Europe and China. This disease can reach epidemic levels and cause extensive damage to grains, especially wheat and barley in humid and semi-humid cereal growing areas of the world, including India, Russia, France, Germany, and the United Kingdom. Particularly, wheat scab or head blight is one of the more damaging diseases of wheat in the United States. Nationwide, this disease has caused the wheat industry millions of dollars in yield losses. In the Midwest and High Plains, wheat scab is the major obstacle to wheat production in recent years. Head blight in addition to attacking wheat also attacks and reproduces on barley, oats, corn, and many other cereals.
This serious plant disease can be caused by several phytopathogens, but primarily by several species in the fungal genus Fusarium. For example, causal agents of the head blight disease in wheat include a number of different Fusarium species, e.g. F. culmorum, F. graminearum (teleomorph, Gibberella zeae), F. avenaceum (teleomorph, G. avenacea), F. poae, as well as by non-Fusarium pathogens such as Microdochium nivale (teleomorph, Monographella nivalis), and Microdochium majus. In the United States, Europe and most agronomically important areas of the world, the predominant causative agent of head blight is Fusarium graminearum (teleomorph, Gibberella zeae sensu strict).
These pathogens typically survive on plant debris. They invade and damage the spikelets of the grain head during flowering, thus preventing or partially impeding the development of grain in the grain head. As a result, the invading scab pathogen can either kill part of the grain head or the entire grain head. Some infected seeds are low in vigor and often fail to germinate. Infected seeds that germinate often die early in the seedling stage due to crown rot or root rot, causing poor stands in the following crop plant. Healthy seedlings can also become infected at emergence. In addition to poor, unthrifty stand, yield losses due to pathogen infestation can be quite high if conditions are favorable for development of the disease.
The fungal pathogens of Fusarium genus spread across grain cultivating areas all over the world, and are known to cause severe damage, particularly in areas with high rainfall between flowering and grain filling. When Fusarium graminearum is the causative agent, this disease is of primary concern because it does not only reduce the commercial values of the contaminated grains, in addition to yield losses, but because Fusarium infection can also lead to the accumulation of trichothecene mycotoxins in the grains thus threatening the health of human and livestock. Trichothecenes are major mycotoxin contaminants of cereals worldwide, causing feed refusal, vomiting, diarrhea and weight loss in non-ruminant animals and posing a health threat to other animals and humans when exposure levels are high. This threat has been exacerbated by the recent shift in the F. graminearum strains in the United States towards greater toxin production and vigor. Most frequently found mycotoxins are deoxynivalenol (DON, also known as vomitoxin) and zearalenone (ZEA). Deoxynivalenol in particular is a very dangerous toxin, causing gastrointestinal disorders accompanied by hemorrhagic conditions and the like in humans and animals that eat infected grains, leading to death in some cases. Since deoxynivalenol is generally stable against changes in pH and high temperature, detoxification can be very difficult. Therefore, grains contaminated beyond a certain level cannot be used in any form of brewing, processing, or livestock feed, and thus often need to be disposed of.
To date, various strategies have been deployed for controlling head blight in crop plants. Promising options include chemical measures, the development of resistant crop cultivars, and traditional practices of crop rotation and tillage of fields. Among these options, chemical pesticides can be somewhat effective in reducing head blight infestation, but residue concerns regarding the use of fungicides late in crop development, typically at flowering stages only a few weeks before harvest, lessen their attractiveness. Advances in developing resistant cultivars using traditional breeding and genetic engineering represent another disease control alternative are also occurring. Reported examples of genetic engineering advances include altering the production of a plant hormone or manipulating the plant hormone signaling pathway. In recent years, considerable advances in the area of traditional breeding have been made in understanding the genetic basis of resistance to head blight disease and a number of genes and quantitative trait loci (QTL) conferring resistance have been reported. However, progress in improving crop resistance to head blight disease has been slow, largely because of the difficulty of studying this disease. In fact, relatively little is currently understood about the mechanisms involved in resistance or susceptibility. In addition, the genetic diversity of Fusarium species, which are the predominant causative agents of the disease, often raises concerns regarding how durable the efficacy of chemical fungicides and resistant cultivars will be. As a result, practically all wheat cultivars currently in large-scale production remain vulnerable to infection.
In addition, although some success in controlling head blight disease can be expected by traditional practices such as plowing fields to bury crop residues infested with causative agent, e.g. F. graminearum, after harvest, conventional tillage of fields after harvesting is not compatible with the soil conservation practice of minimum tillage. Considering the potential of long distance inoculum dispersal and the diverse crops that can act as alternative hosts of the pathogens, crop rotation is often an untenable solution. In addition to the problem of pesticide residues in the environment, reports of pesticide resistance and instances of DON content increases in grain can also be concerns with their use. Further, costs and increasing concerns in the public and private sectors over pesticide residues in the environment and food product safety render this disease control alternative less attractive, and have led to requests for crop cultivation using as little pesticides as possible.
In summary, despite considerable advances in developing techniques to control head blight, reducing the impact of this devastating disease on grain production and quality remains an intractable problem. Therefore, identification and development of new head blight controlling techniques is essential in improving the production and quality of many cereal crops. These problems require urgent solution not only in United States, but also across the globe, including the Asia and Europe.
Biological control of head blight disease has attracted considerable interest since the mid 1990's. Biological control agents (BCAs), though currently very limited in number, could be an environmentally acceptable method for substantially decreasing the level of disease incited by pathogens such as Fusarium. Public acceptance, compatibility with other disease management measures, and durability are among favorable factors in support of developing strategies for biologically controlling head blight disease. Biological control agents could play an important role in organic cereal production. In conventional production, such agents may extend protection of spikes past the flowering stage after chemical fungicides can no longer be applied. To date, significant advances in the area of biological control have been achieved. For example, certain strains of spore-producing bacteria (such as Bacillus and Pseudomonas species) and yeasts (such as Cryptococcus species) show some promise for the control of head blight disease and the reduction of mycotoxin contamination. However, despite these and other advances, the need remains for improved microorganisms for use in the biological control of head blight disease. Although BCAs have become a more acceptable solution for plant pathogens and BCA products have been marketed to a greater extent than ever before, to date there have been few attempts to develop strategies and antagonistic microorganism for biologically controlling head blight disease. Furthermore, the life cycle of Fusarium spp. and other causative agents of head blight disease suggest that the pathogens can potentially be susceptible to biocontrol techniques using antagonistic microorganisms at different developmental stages. Thus, there is a need to identify new biological control agents, preferably with different modes of actions, as well as biocontrol methods that can help effectively prevent or suppress the development of head blight disease.